Leaky house and oversized new HVAC systems?

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Leaky house and oversized new HVAC systems?

We bought the farm, so to speak in the deep South - a 1-1/2 story house built in 1950. We are looking for thoughts on dealing with a very leaky house with historic ductwork and new, oversized HVAC systems.

Upstairs are with two rooms and a bath finished in the old attic space with knee walls insulated with fiberglass batts. The attic space has very minimal blown in fiberglass insulation. The remainder of the exterior walls are, from inside to out - dimensional boards, asphalt paper, an air gap, and brick veneer. The subfloor is similar to the walls - constructed of diagonally placed dimensional lumber with nice oak hardwoods over them. The previous owner insulated the crawl space with fiberglass batts between the joists.

Blower door testing showed we had a whopping 84% natural air exchange (4300 CFM50). Duct blaster testing showed 34% leakage from the ductwork in the crawl space and 20% leakage from the ductwork in the attic.

To top it off the upstairs (about 500 square feet) is served by a 1.5 ton 13 SEER heat pump (2007) with ductwork in the unconditioned portion of the attic. Downstairs (about 1400 square feet) is served by a 3 ton 13 SEER gas pack unit installed in 2010 with very old ductwork in the crawl space that sweats and drips in the summer.

There is too much heating and cooling capacity for the existing house. When we tighten the building envelope, re-insulate the attic and kneewalls, and fix/replace the ductwork, the effect of the oversized HVAC systems will be amplified. The energy auditor cautioned that, if airseal and insulate but don't replace the oversized systems we may end up with mold issues. His mechanical contractor thinks he can ratchet down the existing systems and it might work. He recommended humidity monitoring in the house, especially during the summer and noted that we may very well end up replacing the systems anyway. Moisture levels in the crawl space joists in August were 12-17% and there was no evidence of mold/mildew.

The house is managing the flow or moisture pretty well but not heat and air. I would prefer to control the source of fresh air entering the house. I'd like to replace the leaky crawlspace ductwork to reduce condensation adding to moisture in the crawl space and improve the air quality in the house. I don't have the budget to replace basically new HVAC equipment.

If you had about $10k to spend on this, what would you do?

Asked by Sue Davis
Posted Wed, 12/05/2012 - 17:20